MEM_LIMIT Query Option

When resource management is not enabled, defines the maximum amount of memory a query can allocate on each node.
Therefore, the total memory that can be used by a query is the MEM_LIMIT times the number of nodes.

There are two levels of memory limit for Impala.
The -mem_limit startup option sets an overall limit for the impalad process
(which handles multiple queries concurrently).
That limit is typically expressed in terms of a percentage of the RAM available on the host, such as -mem_limit=70%.
The MEM_LIMIT query option, which you set through impala-shell
or the SET statement in a JDBC or ODBC application, applies to each individual query.
The MEM_LIMIT query option is usually expressed as a fixed size such as 10gb,
and must always be less than the impalad memory limit.

If query processing exceeds the specified memory limit on any node, either the per-query limit or the
impalad limit, Impala cancels the query automatically.
Memory limits are checked periodically during query processing, so the actual memory in use
might briefly exceed the limit without the query being cancelled.

When resource management is enabled, the mechanism for this option changes. If set, it overrides the
automatic memory estimate from Impala. Impala requests this amount of memory from YARN on each node, and the
query does not proceed until that much memory is available. The actual memory used by the query could be
lower, since some queries use much less memory than others. With resource management, the
MEM_LIMIT setting acts both as a hard limit on the amount of memory a query can use on any
node (enforced by YARN) and a guarantee that that much memory will be available on each node while the query
is being executed. When resource management is enabled but no MEM_LIMIT setting is
specified, Impala estimates the amount of memory needed on each node for each query, requests that much
memory from YARN before starting the query, and then internally sets the MEM_LIMIT on each
node to the requested amount of memory during the query. Thus, if the query takes more memory than was
originally estimated, Impala detects that the MEM_LIMIT is exceeded and cancels the query
itself.

Type: numeric

Units: A numeric argument represents memory size in bytes; you can also use a suffix of m or mb
for megabytes, or more commonly g or gb for gigabytes. If you specify a value with unrecognized
formats, subsequent queries fail with an error.

Default: 0 (unlimited)

Usage notes:

The MEM_LIMIT setting is primarily useful in a high-concurrency setting,
or on a cluster with a workload shared between Impala and other data processing components.
You can prevent any query from accidentally using much more memory than expected,
which could negatively impact other Impala queries.

Use the output of the SUMMARY command in impala-shell
to get a report of memory used for each phase of your most heavyweight queries on each node,
and then set a MEM_LIMIT somewhat higher than that.
See Using the SUMMARY Report for Performance Tuning for usage information about
the SUMMARY command.

Examples:

The following examples show how to set the MEM_LIMIT query option
using a fixed number of bytes, or suffixes representing gigabytes or megabytes.

The following examples shows the automatic query cancellation
when the MEM_LIMIT value is exceeded
on any host involved in the Impala query. First it runs a
successful query and checks the largest amount of memory
used on any node for any stage of the query.
Then it sets an artificially low MEM_LIMIT
setting so that the same query cannot run.